1. Field of the Invention
The present invention relates to a heat treatment apparatus, more particularly, to a heat treatment apparatus incorporating improved heating resistive element for heat-treating objective material
2. Description of the Related Art
Conventionally, various heat treatment apparatuses, such as thin-film formation apparatuses (e.g., a chemical vapor deposition apparatus and an epitaxial film growing apparatus), oxide film formation apparatuses, and thermal diffusion apparatuses for doping impurities, have been used in processing semiconductor wafers.
Of these, any conventional diffusion type heat treatment apparatus for performing various heat treatments on semiconductor wafers comprises a process tube loaded with a plurality of semiconductor wafers, a heating resistive element, and a heat insulating member surrounding the heating resistive element.
A spiral heater made of FeCrAl is made available as heating resistive element. The spiral heater can raise the temperature in the process tube to temperatures as high as 1200.degree. C. The heat insulating member is made of, for example, laminated ceramic fibers. The ceramic-fiber insulating member decreases the quantity of heat which is radiated and conducted from the heater, in order that semiconductor wafers can effectively be heat treated.
Since the conventional diffusion-type heat treatment apparatus mentioned above has a spiral heater made of FeCrAl having relatively low density of allowable current, the temperature inside of the process tube is raised at only about 10.degree. C./min. In consequence, it is difficult for the heat treatment apparatus to heat-treat semiconductor wafers at high speed. Furthermore, the heating resistive element deforms much when heated to high temperatures, inevitably contacting the heat insulating member. Upon contacting the insulating member, the element reacts with the insulating material and is subsequently cut.
To enhance the wafer processing efficiency, a heat treatment apparatus has been developed which has a lamp capable of raising the temperature at 50.degree. to 100.degree. C. per second. Nevertheless, when this apparatus is operated, the surface of each semiconductor wafer loaded in the process tube is not uniformly heated, a temperature difference of about 40.degree. C. is detected between the most heated portion and the least heated portion. This results in crystal defects, like slip, in the processed semiconductor wafer.
On the other hand, an electric furnace incorporating heating elements made of molybdenum silicide (MoSi.sub.2) is disclosed in Published Unexamined Japanese Patent Application No. 48-8657 of 1973. Published Unexamined Japanese Patent Application No. 1-155194 of 1989 discloses a heat treatment furnace having a plurality of heaters and heat insulating members which are located in a plurality of heating zones, respectively.